pi_bridge

BEAM runtime bridge for pi and the pi-elixir package. It provides the Elixir-side Pi.* modules used for Livebook-style stateful eval, ExAST-backed structural tools, stdio transport, executable Elixir skills, LLM calls through pi's active model, OTP-backed logical agents, and bidirectional plugin UI events.

pi_bridge is inspired by Vibe: keep the model-facing surface small, but let trusted Elixir code operate from inside the running BEAM.

Runtime model

pi_bridge is bundled and started by the pi-elixir extension as an extension-owned sidecar. Target projects do not need to add :pi_bridge to mix.exs; the sidecar loads the target Mix project context and communicates with pi over stdio.

Public API ergonomics

The public API intentionally separates single-call and orchestration shapes:

Boundary JSON examples are documented in docs/protocol.md.

Eval

Pi.Eval.run/2 is the trusted project introspection path. It evaluates inside the project BEAM with project modules, aliases, application config, OTP processes, Repo modules, and IEx helpers available.

Structured eval from the pi tool is stateful: bindings and Macro.Env are kept in a supervised evaluator and persisted as sidecar snapshots next to the pi session. That gives IEx/Livebook-like continuity across eval calls and resume/branch navigation without inlining large state into JSONL transcripts.

Useful eval helpers:

Pi.Eval.bindings()

Pi.Eval.forget(:large_result)

Pi.Eval.reset()

QuackDB mirror analytics are available through token-efficient aliases in eval:

# preloaded: import Ecto.Query; use QuackDB.Ecto

# preloaded: alias Pi.Self, as: Self

# preloaded: alias Pi.CodeMap, as: CodeMap

# preloaded: alias Pi.Quack, as: Q; require Q

# preloaded: alias Pi.Quack.Event, as: E; alias Pi.Quack.SessionFile, as: SF



Self.status()

Self.context("why did sync crash?", limit: 5)



# Reach-backed semantic reflection after edits.

CodeMap.reflect(changed: true)

CodeMap.hotspots(path: "lib/my_app/module.ex")

CodeMap.context("MyApp.Module.fun/2")



from(e in E,

  group_by: e.tool_name,

  order_by: [desc: count(e.id)],

  select: %{tool: e.tool_name, n: count(e.id)}

)

|> Q.table()

Use Q.score/2, Q.matches/2, Q.json/2, and Q.json_text/2 inside normal QuackDB/Ecto queries for FTS and payload analysis.

For untrusted snippets, use the Dune-backed sandbox:

{:ok, %{inspected: "42"}} = Pi.Eval.sandbox("40 + 2")



# Negative example: restricted system access is blocked.

{:error, message} = Pi.Eval.sandbox(~s(System.cmd("ls", [])))

The sandbox applies timeout, reduction, heap, and allowlist limits. It returns {:error, :unavailable} if the optional :dune dependency is not present.

LLM

pi owns provider/model selection, credentials, streaming, cancellation, usage, and transcript UI. The BEAM side sends structured completion/stream requests over the active bridge; it does not create a separate provider stack.

{:ok, text} = Pi.LLM.complete("Explain this module")



stream = Pi.LLM.stream("Draft a migration plan")

Enum.each(stream.stream, &IO.write/1)

ReqLLM can route through the active pi session as an adapter on top of that pi-owned model path:

Pi.ReqLLM.install()

ReqLLM.generate_text(Pi.ReqLLM.current_model(), "Summarize the current project")

Pi.ReqLLM.current_model/0 returns ReqLLM's inline model struct for the active pi session. Use it instead of the string "pi:current" so ReqLLM does not try to verify the dynamic local route against its public model catalog.

Feature flag:PI_ELIXIR_LLM=0 disables BEAM-initiated LLM requests.

Sessions and agents

The bridge keeps one pi Node.js/TUI process and one embedded BEAM process. Subagents are not extra pi processes; they are lightweight OTP session workers supervised inside BEAM:

pi Node.js/TUI

  └─ embedded BEAM

       ├─ Pi.LLM.Broker

       └─ Pi.Session.Supervisor

            ├─ Pi.Session.Worker

            └─ Pi.Session.Worker

Use Pi.Session when you need attachable, subscribable session state:

{:ok, root} = Pi.Session.start(name: :root)

{:ok, reviewer} = Pi.Session.child(root, name: :reviewer)

{:ok, "done"} = Pi.Session.run(reviewer, "Review this change")



{:ok, state} = Pi.Session.subscribe(reviewer)

Session snapshots are emitted as pi_session events. The extension renders active/running work as a compact live widget, then emits completed root session trees once as inline transcript entries (elixir-sessions). Active BEAM snapshots are reloaded directly from the bridge on session start. Private slash commands control active sessions without adding model-facing tools. The TUI accepts either id=session_123 or the raw session_123 as the command argument:

/elixir:sessions.cancel id=session_123

/elixir:sessions.rerun id=session_123

Snapshots carry structured fields such as prompt/response previews, current activity, recent streaming output, run_count, completed_at, and timing. Streaming session runs can emit :delta events before the final assistant message:

{:ok, text} = Pi.Session.run(session, "Draft notes", stream: true)

Feature flag:PI_ELIXIR_SESSIONS=0 disables session snapshot/control affordances.

Use Pi.Agent for convenience orchestration over those sessions. Agent helpers use canonical %Pi.Session.State{} values and runtime Pi.Session workers; there is no separate agent session registry:

{:ok, result} = Pi.Agent.run("Review this change", name: :reviewer)



{:ok, run} =

  Pi.Agent.chain([

    "Draft an implementation plan",

    "Review the plan for risks"

  ])



{:ok, fanout} = Pi.Agent.fanout(["Review tests", "Review API", "Review docs"])

For supervised delegation, start jobs. A job owns lifecycle; its child Pi.Session owns the transcript:

{:ok, job} = Pi.Agent.start("Review this module", role: :reviewer)

job.status

#=> :running



{:ok, done} = Pi.Agent.await(job, 60_000)

done.status

#=> :done



{:ok, text} = Pi.Agent.result(done)

Pi.Session.state(done.child_session_id)

Run multiple jobs when the tasks are independent:

{:ok, jobs} =

  Pi.Agent.run_many([

    %{task: "Review tests", role: :reviewer},

    %{task: "Review API", role: :reviewer},

    "Review docs"

  ])



Enum.map(jobs, &Pi.Agent.await(&1, 60_000))

Attach jobs to a parent session when you want parent-visible lifecycle events in the session widget:

{:ok, parent} = Pi.Session.start(name: :review)

parent_id = Pi.Session.state(parent).id



{:ok, job} = Pi.Agent.start("Review tests", role: :reviewer, parent_session_id: parent_id)

{:ok, done} = Pi.Agent.await(job, 60_000)



Pi.Session.state(parent).events

# includes :agent_job_started and :agent_job_finished

Cancel long-running work through the job lifecycle handle:

{:ok, job} = Pi.Agent.start("Explore a risky option", role: :researcher)

:ok = Pi.Agent.cancel(job)

{:error, cancelled} = Pi.Agent.await(job, 100)

cancelled.status

#=> :cancelled

Pi.Agent.run/2 keeps the single-run shape {:ok, %Pi.Agent.Result{}} | {:error, %Pi.Agent.Result{}}. chain/2, parallel/2, and fanout/2 return {:ok, %Pi.Agent.Run{}} | {:error, %Pi.Agent.Run{}} so orchestration metadata and partial results are explicit. Job APIs return %Pi.Agent.Job{} lifecycle handles with status, result, error, parent_session_id, and child_session_id.

Plugin command/event/hook lifecycle

  1. On stdio startup, BEAM sends ready with plugin command inventory.
  2. The TypeScript extension registers each plugin command as /elixir:<name>.
  3. Running the slash command sends pi_plugin_command to BEAM and dispatches handle_command/3.
  4. Pi.Plugin.Event.emit/2 sends {type: "event"} back to pi and is published on pi.events.
  5. Before a pi tool executes, the extension calls pi_plugin_tool_call; plugin tool_call/3 may block or return an input-only patch.
  6. After a pi tool result, the extension calls pi_plugin_tool_result; plugin tool_result/3 may patch result content or isError.
  7. Malformed hook payloads are rejected before plugin callbacks run.

Session bridge APIs

BEAM code can ask the pi extension for small session-state snapshots, persist branch-aware custom entries, or emit a visible custom transcript message:

{:ok, info} = Pi.Host.info()

{:ok, %{tools: tools}} = Pi.Host.active_tools()

{:ok, "ok"} = Pi.Host.append_entry("demo-state", count: 1)

{:ok, "ok"} = Pi.Host.send_message("demo-message", count: 1)

Plugins

Feature flags:PI_ELIXIR_PLUGINS=0 disables built-in/project-local plugins, hooks, UI events, and plugin commands. PI_ELIXIR_SKILLS=0 disables executable skill discovery.

Built-in optional plugins are loaded before project-local plugins. The built-in DuckDB event mirror (Pi.Mirror.QuackDB) is enabled by default; set PI_ELIXIR_MIRROR=0 to disable it. By default it writes ~/.pi/elixir/session-mirror.duckdb; override with PI_ELIXIR_MIRROR_DB, or point at an existing Quack server with PI_ELIXIR_MIRROR_QUACKDB_URI and PI_ELIXIR_MIRROR_QUACKDB_TOKEN.

Project-local plugins live in priv/pi_plugins, .pi/plugins, or pi_plugins. Each plugin is isolated behind a Pi.Plugin.Worker process.

defmodule DemoPiPlugin do

  use Pi.Plugin



  def init(_opts), do: {:ok, %{events: 0}}



  def handle_event(_event, state), do: {:noreply, Map.update(state, :events, 1, &(&1 + 1))}



  command name: :demo, description: "Run the demo plugin command"



  def handle_command(:demo, args, state), do: {{:ok, "demo #{args}"}, state}



  # Negative example: block a tool call.

  # Return {:block, reason} to prevent a tool call, or {:ok, patch} to merge into the tool input only.

  def tool_call(%{"toolName" => "bash"}, _context, state), do: {{:block, "bash blocked"}, state}

  def tool_call(_call, _context, state), do: {:ok, state}



  # Return {:ok, patch} to patch a tool result. Supported TypeScript-side patches include

  # string `content` and boolean `isError`.

  def tool_result(%{"toolName" => "demo"}, _context, state) do

    {{:ok, %{"content" => "patched by plugin"}}, state}

  end



  def tool_result(_result, _context, state), do: {:ok, state}



  def apis do

    [name: :demo_plugin, module: __MODULE__, alias: :DemoPlugin]

  end

end

Examples

See examples/vibe_workflow.exs and examples/demo_plugin.exs.